What does the study say?
The study, based on data from double jabbed participants who had a prior infection, found a "very large drop" in how well the vaccine's antibodies neutralised the new strain, compared to an earlier variant.
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However, WHO's Dr Mike Ryan said there was no sign Omicron would be better at evading vaccines than other variants.
“We have highly effective vaccines that have proved effective against all the variants so far, in terms of severe disease and hospitalisation, and there’s no reason to expect that it wouldn’t be so” for Omicron, Dr Ryan, the WHO's emergencies director, told AFP news agency.
He also said that initial data suggested Omicron did not make people sicker than the Delta and other strains: "If anything, the direction is towards less severity," he said.
The South African study, which has not yet been peer-reviewed, tested blood from 12 people given two doses of Pfizer and found the Pfzier/BioTech vaccine may result in up to 40 times fewer neutralising antibodies against Omicron than the original Covid strain.
Professor Alex Sigal, a virologist at the Africa Health Research Institute, said that Omicron's ability to escape vaccine antibodies is "incomplete".
Professor Sigal also said the results from 12 people were "better than I expected of Omicron".
What other protection is there?
His study also suggested that a combination of vaccinations and prior infection could still neutralise against Omicron, suggesting boosters may bring some significant benefit.
More data on how well the Pfizer jab works against the variant is expected to be released within days.
There is no significant data yet on how well the Moderna, Johnson & Johnson and other jabs hold up against the new Omicron.
A version of this article originally appeared on NationalWorld.com